I have a hard time manual focusing my Canon EOS 350D.
Compared to the few higher spec DSLRs that I played with, and that have a pentaprism, my Rebel's (pentamirror) viewfinder feels crammed and somewhat darker.
I feel like I have to squint in order to figure out if the subject is in focus.
It also has a simple matte focusing screen, so it doesn't help figuring out when critical focus is achieved either.

I remember having used my dad's all-manual Praktica film camera in my teen years, and at the time I felt no burden with manual focus (exposure, now, that's a different story...)

I think the focusing screen on that camera was a split image + microprism setup, something like this:

Why do (most of?) the DSLRs of today not use such focusing screens?
Do the manufacturers work under the assumption that most focusing is AF (which is probably true)? But still, how would it hurt to have a split image as a focusing aid?

Is there an obvious downside to such focusing screens, that I am missing?

For the sake of completeness, I have found two places that sell replacement focusing screens for DSLRs. I'm sure there are others.

This is the 1st one, I think based in Taiwan. They have a great comparison of different types of focusing screens (that's where the above image links go, too).

5 Answers
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Split-prism focusing depends on your using fairly fast lenses. When the normal "kit" lens was a 50/1.8 (or 50/1.7, or something similar) that worked well. With slower lenses, one side or the other (or both) will be "blacked out" nearly all the time, and it provides no help in focusing. A typical kit lens nowadays is a zoom with a maximum aperture of something like f/4.5 of f5.6, which is too slow for a typical split-prism to work well at all.

You can design a split prism to work with slower lenses, but the slower of lens you design it for, the less precise it becomes (the less offset you see between the two sides as the image goes out of focus). I've never tried one that was designed for something like f/5.6 so I can't say for sure, but at a guess the offset would be small enough that it wouldn't be very helpful.

Microprisms lose effectiveness with slow lenses in much the same way (when you get down to it, they're basically a large number of much smaller split prisms).

There is a third possibility. Most current focusing screens don't diffuse the light very much as it passes through the screen. The less it diffuses the light, the less apparent blurring is as the picture goes out of focus. If you use a screen that diffuses the light more, it'll give a better indication of how well the picture is focused.

This has a shortcoming too though: diffusing the light more also makes the "picture" in the viewfinder dimmer. Much as with the others, you're stuck with tailoring the focusing screen to the lenses you're going to use. Professional level cameras that they expect to be used (at least primarily) with fast lenses will also typically have focusing screens available that diffuse the light more, and therefore give a better indication of focusing. The trade-off of that is that the view they give is dimmer, so you nearly need to use them with relatively fast lenses.

I replaced my focus screen in my old T2i with a split-prism and it was significantly darker (in the already dark pentamirror finder). Past f/5.6 it was nearly useless on most lenses but some are better than others.
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Ron WarholicMay 4 '11 at 23:24

For the anonymous editor: when you're focusing the camera, it'll normally keep the lens at the widest aperture. Then, just before opening the shutter, it flips up the mirror and closes the diaphragm to the selected aperture, so unless you're using a Dept of Field preview (which the Sears KS-2 probably lacks) you're not looking through the stopped-down aperture while focusing.
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Jerry CoffinSep 25 '12 at 20:19

Split-prism focusing screens went away with AF. I have a Nikon F90 from the early 90s with a plain matte screen (and only one focus area). I can't speak for other brands and models, AF film bodies being the red-headed stepchildren of the used gear market. They're cheap because no-one wants them.

I think the reasons for not having them are both usability - a plain screen allows you to see the AF focusing patches (which you're assumed to be primarily using) and economic. A matte screen is less complicated to manufacture - just cut from a larger sheet! No need to calibrate and center etc. Also, your manual focus lenses just became harder to use - why not get one of these new cool AF lenses?

Regarding third-party screens: I recently purchased a Katz Eye screen for my D200. It was pricey, but worth it! Maybe the D200 has a slightly larger viewfinder than other DSRLs, but I now find focusing manually much easier.

For entry level camera - it's likely not going to end your manual focusing misery. It may help, but the viewfinders are just much smaller than some of their full frame or classic film brothers. That size hurts, alot.

Reading up on focusing screens shows that it can throw off exposures values by a half stop or so in some cases.

UPDATE: The below appears to be incorrect. Thanks RBerteig for the correction.

The Canon EOS 7D has an active focusing screen that changes its marking according to the current mode (I assume 60D and maybe T3i are same). It is probably impossible to keep this functionality with a split prism screen.

No, the reticule markings are provided by an LCD placed just above the focusing screen. Both Nikon and Cannon make DSLRs where the screen can be replaced without damage to or loss of the LCD reticule and other in-viewfinder displays.
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RBerteigMay 4 '11 at 22:53

I researched this shortly after buying a Nikon D70s, but never got around to buying the replacement screen. I'm toying with it again for my D90, but still haven't bothered. Nikon at least sells a couple of different screen options, and there are third parties as well.
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RBerteigMay 4 '11 at 23:14